Adenovirus vectors (Ad) are the most common viral vector type used in clinical studies worldwide. Upon intravascular delivery, Ad elicits multifaceted host innate immune and inflammatory responses that drastically compromise both the safety and the efficacy of the Ad-based therapy due to a dose-limiting systemic toxicity. The molecular and cellular mechanisms governing this systemic anti-Ad inflammatory response remain poorly understood. By analyzing the initiation of inflammation in vivo after the injection of capsid-modified Ad vectors into mice knockout for critical inflammatory mediators, we have made an original observation that Ad interaction with 23 integrins and macrophage cell-surface receptors triggers a unique inflammatory pathway mediated by IL-11. Activation of this pathway initiates a downstream cytokine and chemokine cascade that depends on functional IL-1RI signaling. However, our studies also indicate that, in addition to this cytokine production, intravenous Ad administration induces a rapid pro-inflammatory MF death and the influx of pro-inflammatory leukocytes into affected sites. The Specific Aims of this proposal are designed to define the contribution of each of the known components of Ad-induced inflammation into the clinically relevant systemic toxicity observed after the intravenous Ad administration. In Specific Aim 1, we will investigate the molecular mechanisms involved in mediating Ad-induced pro- inflammatory macrophage cell death in vivo, and its role in activating systemic anti-Ad inflammatory response. In Specific Aim 2, we will analyze the phenotypic markers and functional activation states of inflammatory leukocytes, accumulating in the spleen and liver after intravenous Ad injection, and their role in mediating the acute systemic anti-Ad inflammatory response. In Specific Aim 3, we will construct novel Ad vectors that would avoid interaction with 23 integrins and analyze their gene delivery and systemic toxicity profiles in vivo. These studies will improve our understanding of the fundamental mechanisms of host defense against viral pathogens and may ultimately lead to the development of safe and effective Ad vectors for the therapy of a wide range of inborn and acquired human diseases.